Motivation

The application of wireless sensor networks to the underwater domain
has huge potential for monitoring the health of river and marine
environments. The oceans alone cover 70% of our planet and along with
rivers and lakes are critical to our well-being. Monitoring these
environments is difficult and costly for humans: divers are regulated in
the hours and depths at which they can work, and require a boat on the
surface that is costly to operate and subject to weather conditions. A
sensor network deployed underwater could monitor physical variables such
as water temperature and pressure as well as variables such as
conductivity, turbidity and certain pollutants. The network could track
plumes of silt due to dredging operations or pollutants flowing in from
land, and it could monitor and model the behavior of underwater
ecosystems.

However a number of problems confront us in achieving this goal. Some
such as power efficiency, deployment and repair are common to wireless
sensor network deployments on land, though perhaps more difficult in the
underwater environment. Other issues, however, render the problem
radically different. A key issue is communications. Current terrestrial
wireless sensor network applications to date has used radio. At all
frequencies radio waves are attenuated so strongly in salt water that
radio communications is impractical. From the electromagnetic spectrum the
visible light is much less attenuated especially at blue-green side of the
spectrum. Acoustic communication is the other feasible method of
communication in underwater environment. While the data rate of optical
communications can be significant the range is limited by the the water
turbidity and attenuation to meters.

Approach

Our sensor network contains static and mobile nodes. All nodes contain
flash memory for Each static nodes is build around a CPU unit developed by
CSIRO called a Fleck, based on the Atmega128 processor, with
128kbyte of program flash memory, 4kbyte of RAM, and 512kbyte of flash
memory for data logging/storage. The Fleck is interfaced to a special
optical communications board through 2 digital IO pins. One of these pins
is used to turn an LED on or off, while the other is used to sense the
output from a matched photodiode. All the analog electronics (e.g.,
amplifiers etc) are on the interface board. The Fleck is also interfaced
with a sensor board. All these boards are connected together in a stack
using stack-through connectors. The underwater sensor node is contained in
a yellow watertight Otter box which measures 170 x 100 x 90mm and
has been modified to incorporate the sensing and communication hardware.
The Otter box is guaranteed to be watertight up to a depth of 30 meters.
Each box has a high speed optical communication module that uses 532nm
light, and is capable of a range of 2.2m (can be increased to 7 meters by
adding lenses on one of the devices and actively pointing it toward to
other), within a cone of 30 degrees and a maximum data rate of 320kbits/s.
Additionally, there is an acoustic communication module using 30kHz FSK
modulation with a range of 20m omnidirectional, and a data rate of
50bit/s. The same module is also used for ranging (Two sensor nodes can
determine the distance between them using time of flight of the sound
waves). For sensing, each node has a pressure sensor, temperature sensor,
and a CMUCam camera capable of color pictures with a 255 x 143 resolution.
The top side of the box contains a 170 mm rod with an LED beacon, which an
AUV can use to locate the box, dock, and pick it up. Future versions will
contain a XENON flash tube for increasing the distance for reliable node
location to about 20 meters. The sensor node is powered by 3 alkaline C
cells. Three C cells can provide 27 wh and four days of continuous
operation with all sensors and communication hardware fully powered. The
box is weighted to be 40% negatively buoyant, and balanced such that if
dropped in water it always lands top up.

Figure 1. Several sensor nodes and AMOUR
mobile node.

Progress

We constructed 20 static sensor nodes (Aquaflecks) in colaboration with
the CSIRO team in Australia. We also equiped two different AUVs (the CSIRO
Starbug and our AMOUR) with optical and acoustical communication to be
part as the mobile sensor nodes. We conducted experiments in CSIRO pool in
Brisbane Australia as well as in MIT tow tank and Charles river in Boston.
We demonstrated optical communications achieving 320kbits/s data rate over
2.2 meters range in clean water. We demonstrated our low cost acoustical
communications achieving 50bits/s at 15 meters, as well as the distance
measuring capability obtaining less the 2% error up to 15 meters.

Using the Starbug mobile node we demonstrated autonomuous data muling from
a sensor network consisting of 6 sensor nodes placed in a grid. Starbug
visited one by one the nodes, and colecting their data. Navigation between
the nodes was basted on the magnetic compass.

Using AMOUR mobile node we demonstrated the ability to autonomuously
dock with a static node, to pick it up and relocated it. Amour can locate
and dock with a static node from a distance of 4 meters with a cone of 90
degrees.

Future

As future work we want to work towards a full deployment of an
underwater sensor network in the ocean environment, that will enable a
full test of our concepts in a real environment. This will imply the AUVs,
fully autonomuosly, deploying of the static nodes precisely in a specified
locations. The nodes will remain there and collect data, while our
acoustic communication and localization protocols can be tested. The
mobile nodes (AUVs) will conduct experiments like collecting data from an
interesting spot detected by the sensor network. The sensor network will
also provide navigational aid to the AUVs throughout the experiment using
their acoustic self localization and acoustic localization of the mobile
nodes. The AuV will collect the data from the static nodes, using the high
speed optical comms. When necessary the AUVs will replace the defective
and the low battery nodes in an autonomuous fashion.